The demand for inexpensive, high quality, easily produced ultrapure water is increasing in a number of industries. In the semiconductor industry for example, as the scale of microchip semiconductor devices continues to decrease, there is an increasing need for reduction of residual contaminants in process water. Conventional standards for ultrapure water are deionization to a resistivity of 18 megohms, absence of particles greater than 0.20 micron, and the presence of fewer than 1.0 bacterial colonies per milliliter.
FIG. 1 is a simplified schematic diagram of a conventional large-scale industrial water ultrapurification system 10. Because source water 12 may come from wells, surface runoff, or municipal supplies, and may contain a variety of metals, organic compounds, silts, or other contaminants, scheme 10 employs a variety of processes and components to purify source water 12 to the desired water quality. Source water 12 generally undergoes multistage processing through a pretreatment stage 14, a reverse osmosis (RO) treatment stage 16, and a post-treatment stage 18 before reaching a usage stage 20.
Pretreatment stage 14 is typically designed to remove contaminants that may have deleterious effects on some of the downstream purification processes that may contain more sensitive or expensive components. For example, metal contaminants, such as iron, copper, aluminum, manganese, sodium, or calcium, are known to cause a variety of problems throughout a water ultrapurification scheme.
The most common metal contaminants, iron and manganese, are generally treated by aeration, chemical oxidation, or media filtration. The presence of iron in water to be purified is especially troublesome, as hydroxides formed during oxidation for its removal tend to build up on anionic deionizing resin beds, and may slough off such beds without warning and be carried downstream to the point of use, thereby substantially interfering with the manufacturing process. The pretreatment stage 14 shown in FIG. 1 oxidizes contaminant metals in source water 12 by adding a pH modifier 22 and an oxidizing agent 24, such as chlorine. The oxidized metals precipitate out of solution and are subsequently removed by large particle filters 26.
Because oxidizing agents 24 also pose problems to many downstream process components, in order to remove them from the water treatment systems, a reducing agent 28, such as sodium bisulfite, may be subsequently added to the oxidized source water 12, or the source water 12 may be passed through granular activated carbon (not shown), before it is conveyed to the RO membranes treatment stage 16. A valve 30 regulates the source water flow through a heat exchanger 32 employed to increase the temperature of the source water 12 to about room temperature to facilitate its flow through the RO treatment stage 16. Source water 12 may also be subjected to a set of prefilters 34 before it is finally conveyed by a high pressure pump 36 to the RO treatment stage 16.
State-of-the-art water ultrapurification schemes have recently begun to employ RO membranes 40 and 42 in double pass configurations, where each pass is typically arranged in a series array where the permeate of the prior membrane is stored, then pumped as the feed to the next set of membranes. The reject water or retentate from each later pass through an RO membrane is typically recycled back to the feed side pump, repressurized, and then to the feed side of the earlier RO membranes. The RO treatment stage 16 in FIG. 1 depicts such a double pass configuration through the two sets of RO membranes 40 and 42.
Pump 36 forces pretreated feed water 44 through a set of RO membranes 40 at a pressure in the range of 300 to 500 psi, the first pass permeate 46 being at a reduced pressure relative to the feed side pressure and is then directed to a storage tank 48 that is blanketed with nitrogen from nitrogen source 50 to prevent the dissolution of oxygen, carbon dioxide or other gaseous contamination. The first pass retentate containing filtered impurities 52 is emptied into a sewer 54 or, after detoxification, into a National Pollution Discharge Elimination System (NPDES)-permitted outfall.
A pump 56 repressurizes and directs the permeate of RO membranes 40 from storage tank 48 as feed to a second set of RO membranes 42. The second pass RO permeate 66 is directed to a second storage tank 68 that is also blanketed with nitrogen from nitrogen source 70. The second pass RO retentate 72 is recycled to the feed side 76 of pump 36, and thence to RO membranes 40. Each pass of a conventional pre-RO-filtration scheme 10 utilizes a storage tank and a pump and tends to remove about 90% of the salts from the water; in combination with the double pass RO system shown in FIG. 1, about 99% of the salts are removable from the water, and the transition metal ions are removed to a parts-per-billion level. However, there is still a need in the production of ultrapure water for a degree of removal of such metal ion contaminants to a still lower parts-per-trillion level.
Cellulose acetate (CA) is the most commonly employed RO membrane type and is used in flat sheet, spiral-wound and hollow fiber ("HF") configurations. However, polyamide and polyimide (collectively referred to as "PA") thin-film composite ("TFC") and PA HF membranes have several advantages. PA TFC membranes generally exhibit higher rejection and greater flux than most RO membranes. Unfortunately, the chemical makeup of RO membranes, including CA and PA, makes them highly susceptible to oxidizing agents used in RO pretreatment, such as chlorine, even at very low levels. For example, CA membranes can tolerate up to 1 ppm chlorine, but PA membranes typically can tolerate only up to 0.1 ppm chlorine. Therefore, any oxidizing agent 24 used upstream of the RO membranes 40 and 42 must be virtually completely neutralized by the reducing agent 28 before reaching the RO filters. The standard approach of adding sodium bisulfite to neutralize the chlorine typically requires the downstream removal of the added sodium. However, removal of chlorine upstream of the RO membranes 40 and 42 leaves them susceptible to biofouling by micro-organisms. Thus, some water purification schemes 10 purposely leave 0.3 to 0.5 ppm residual chlorine in the upstream water, which is, of course, incompatible with the use of PA RO membranes. Conventional purification schemes must therefore carefully regulate the concentrations of chlorine and bisulfite during water pretreatment because small shifts in their concentrations can degrade either RO membranes or the quality of the product water. Understandably, the constraints imposed by such delicate balancing of chemical concentrations tend to weigh against the use of the more efficient PA TFC and HF membranes.
After RO treatment stage 16, RO product water 80 is conveyed by pump 82 to resin beds 84 in the post-treatment stage 18. Residual chlorine from RO treatment stage 16 is treated before it reaches resin beds 84, since contact with beds 84 may result in the sloughing off of organic materials from the resin, which in turn can result in unacceptably high total organic content (TOC) levels in the ultrapure product water. High TOC levels in the water used to manufacture semiconductors lead to wafer contamination and reduction of yield.
Further post-treatment of ultrapure water may employ a microfilter 86 to capture microorganisms and resin particles, an ultraviolet (UV) sterilizer 88 to kill any microorganism not removed by microfilter 86, and a submicrofilter 90 to remove any residual cellular debris. Post-treated water 96 is then directed to a storage tank 98 that is also blanketed with nitrogen from nitrogen source 100 until water 96 is needed at usage stage 20. When post-treated water 96 is desired for use in manufacturing, it is conveyed by pump 102 through polishing resin beds 104 and another UV sterilizer 106 that is positioned between secondary microfilter 108 and secondary submicrofilter 110 before reaching usage stage 20. Unused post-treated water 96 is returned to storage tank 98.
Because pretreatment additives used in conventional ultrapurification systems often result in deleterious effects downstream in connection with further water treatment, an alternative, less deleterious pretreatment process would greatly simplify water purification schemes.
An object of the present invention is, therefore, to provide a simplified and efficient water ultrapurification scheme.
Another object of the invention is to provide a system and method for removing metals from source water.
A further object of the invention is to provide such a system and method that does not employ oxidizing agents.
These objects and others are met by the present invention, which is summarized and described in detail below.